1. Field of the Invention
The present invention relates to anti-submarine warfare (ASW) technologies and more particularly to a non-acoustic technique for remotely sensing the presence of submerged submarines from an aircraft or a surface ship.
2. Description of the Prior Art
The detection and localization of submerged submarines has traditionally has been dependent primarily on passive acoustic techniques and or magnetic anomaly detection. A major disadvantage of acoustic techniques is the loss of detection capability with advances in the development of noiseless or "silent" submarines. Another disadvantage is the cost and time required to install and maintain large numbers of submerged acoustic sensing devices over wide areas of the ocean in order to achieve the objective.
A submarine detection technique that avoids this disadvantage uses the internal waves created by a moving submarine as the sensed parameter. Internal waves are subsurface waves found between layers of water of different density or within ocean layers where vertical density gradients exist. [see "The Encyclopedia of Oceanography," Vol. I, Rhodes Fairbridge Edition, pages 402-408 (Reinhold Publishing Corp., New York, 1966)]. One way to measure such internal waves is to construct a profile of ocean water temperatures as a function of depth. A technique employed in the past for measuring such profiles involves the use of many temperature sensing elements (thermistors) spaced on a cable and towed by a ship. This is costly, time consuming and generally unsuited to high spatial and temporal resolution coverage of large ocean areas. Furthermore, the technique cannot be used for mapping ocean temperature profiles, that is, temperatures over wide areas of water, because the measuring time is too long compared to the time over which sea temperatures vary.
Another method of measuring ocean temperature-depth profiles is described in U.S. Pat. No. 4,123,160 in which a laser beam illuminates the sea water and observations are made of the Raman scatter from the monomer and hydrogen bonded polymeric forms of water, the ratio of which is a function of temperature. This technique is vulnerable to interference from high background illumination, such as sunlight, because of the relatively wide optical bandwidth of the Raman scattering. Furthermore, in this method there is differential absorption over the Raman band as light transits the water column. Depolarization effects of the water column also limit the effectiveness of the technique when polarization spectroscopy is employed.
Still another laser remote sensing method has been used in limited experiments, see "SPEED OF SOUND AND TEMPERATURE IN THE OCEAN BY BRILLOUIN SCATTERING" by Hirschberg, et al., Applied Optics, August 1984, pages 2624-2628, inclusive. This method relies on the wavelength shift associated with Brillouin scattering from the water. This shift, however, is small so that extremely high resolution is required in optical measurement of the wavelength shift. Typically a Fabry-Perot interferometer is used to resolve the Brillouin shift. However, an interferometer requires a well collimated light source which generally is incompatible with remote sensing applications where, because of spreading, light must be collected from a much larger field of view than is possible with an interferometer.
This invention is directed to submarine detection by the measurement of sub-surface ocean temperatures while avoiding the foregoing disadvantages.
A general object of the invention is the provision of a method of sub-surface submarine detection by remotely and rapidly measuring ocean temperature profiles without interference from high background illumination, such as sunlight.
Another object is the provision of a method of sub-surface submarine tracking by remotely measuring ocean temperature profiles without the need for a precision interferometer.
A further object is the provision of such a method that permits submarine detection long after the vessel has passed the search area.